There are a couple of things I can think of that may be going on:
1: You may not have enough points for the item and have the Show only items I can use (or whatever the checkbox is labeled) checked.
2: Eventhough I've never seen it with LDoN merchants, every once in a while a merchant gets bugged and looses 1 or more standard items, which come back after a server reset.
3: If you can remember anything about the items you were looking at, this
site has a fairly update search of all the LDoN merchant items, you can verify that it is a regular item.
4: Merchant may have been bugged with stuff that was not suppose to be in the inventory.
I can't think of anything else atm, but maybe someone else has an idea. I don't spend much time at LDoN merchants, so there may be other issues I'm unaware of.